Porous polytetrafluoroethylene (PTFE) materials, including expanded PTFE (ePTFE), are described in a number of patents such as U.S. Pat. No. 3,953,566 to Gore, U.S. Pat. No. 5,476,589 to Bacino, and U.S. Pat. No. 7,306,729 to Bacino et al. These porous PTFE materials represent a family of valuable engineered material useful in numerous applications in widely varied industries such as aerospace, automotive, chemical processing, filtration, medical devices, protective clothing and alternate energy, to name but a few of the possible product applications.
In some of these product applications, ePTFE and other porous PTFE materials are treated to enhance or impart additional properties specifically tailored for the targeted application. For example, ePTFE, which is inherently hydrophobic, and thus is not ideally suited for application requiring compatibility with aqueous solutions, can be treated or coated to render the node and fibril microstructure of the ePTFE material hydrophilic. U.S. Pat. No. 5,130,024 to Fujimoto et al., U.S. Pat. Nos. 5,354,587 and 5,209,850 to Abayasekara, U.S. Pat. No. 5,874,165 to Drumheller, U.S. Pat. No. 7,923,054 to Dutta et al., US Application 20090191398 to Moore, US Application 2013066932 to Zheng and WO2009013340 to Hoving et al., are exemplary patents and patent applications primarily directed towards increasing the surface energy of ePTFE by using a minimal amount of hydrophilic material so as not to change the porosity & and fluid permeability appreciably. Such hydrophilic treatments discussed in the prior art do not have significant effect on the mechanical properties of the ePTFE.
In other teachings ePTFE has been rendered oleophobic by treating the node and fibril structure with fluorinated materials to further lower the surface energy. U.S. Pat. No. 5,972,449 to Chung, U.S. Pat. No. 6,074,738 to Von Fragstein et al., U.S. Pat. No. 8,075,669 to Meindl, EP 1,527,335 to Agarwal, WO2006127946/US Pat. Publn. 20070272606 to Freese and EP 1,754,528 to Deyoung are some examples of these oleophobic treatments of ePTFE. Here too, the primary goal of these patents is to lower the surface energy of the ePTFE by using minimal amounts of the oleophobic material so as not to change the porosity and fluid permeability. These oleophobic treatments are not intended to change the mechanical properties of the ePTFE significantly.
So far not much has been reported in the prior art regarding enhancing the mechanical and thermal properties of ePTFE while retaining the porous characteristics of the ePTFE material. As mentioned earlier, ePTFE is an advantageous material and is used in a wide variety of industrial and commercial applications ranging from protective clothing to medical devices to battery separators to filtration.
In the past, few approaches have been used to improve mechanical properties of ePTFE. In one example, Burger et al. in U.S. Pat. No. 6,127,486, taught the creation of a co-continuous micro-porous structure by using a blend of a thermoplastic polymer with PTFE in order to make ePTFE resistant to mechanical degradation by gamma radiation. A challenge with this method is that the thermoplastic needs to be capable of surviving the high temperature processing involved in making of ePTFE. Other examples of efforts to improve mechanical properties in the art include U.S. Pat. No. 4,949,284 to Arthur and U.S. Pat. No. 6,218,000 to Rudolf et al., which describe the use of ceramic fillers in ePTFE processing to improve properties such as dimensional stability and abrasion resistance, respectively. In these cases, the discrete fillers are entrapped within the node and fibril structure of the ePTFE. A limitation of this approach is that only fillers that can survive the high temperature ePTFE processing can be used. In addition, the potential for particulation and contamination from the filler is not desirable in many applications, such as in medical and electronic devices. Various other approaches have been used to improve mechanical properties of ePTFE. For example, U.S. Pat. No. 6,451,396 to Zumbrum teaches the improvement of flex endurance and U.S. Pat. No. 6,737,158 to Thompson teaches the improvement of resistance to fracture by filling the pores of the ePTFE with suitable polymer matrices. However, the resulting composites add significant mass relative to the ePTFE material alone. Also, these materials are not described as possessing adequate “through” porosity and thus are not permeable to fluids. These approaches therefore cannot be used where air and other fluid (e.g., moisture vapor, gas, water, etc.,) permeability is a requirement.
A need exists for improved ePTFE materials with enhanced mechanical (e.g. abrasion resistance, creep resistance, compression resistance, etc.) and thermal (e.g., shrinkage upon heating) properties without adding substantial mass to the ePTFE material or adversely affecting the porous characteristics of the ePTFE material. Such enhanced ePTFE materials can be valuable by enabling additional application possibilities in diverse industries particularly where size, space or weight is a constraint.
Therefore, there continues to be a need for creating porous, air permeable ePTFE composites with improved mechanical and thermal properties without the limitations mentioned above.